The present invention relates to a liquid crystal display device using a main-chain polyester liquid crystal polymer as an orientation layer and having uniform orientation and good memory characteristics.
Generally speaking, a liquid crystal is a meso-material between liquid and solid states which has the flow characteristics of a liquid and the optical properties of a solid. A liquid crystal can change its optical anisotropy by means of an electric field or heat being applied thereto. A liquid crystal display (LCD) device using these characteristics is one type of flat panel display device, and includes a plasma display device and an electro-luminescence display device.
The limitation of the response time of the conventional twisted nematic liquid crystal is 10 ms-20 ms. A ferroelectric liquid crystal (SmC* phase, SmH* phase, etc.) having a faster response time by improving the problem, and a liquid crystal panel using this was reported in 1980 (Applied Physics Letters, Vol. 36, p899) by N. A. Clark and S. T. Lagerwell (see also U.S. Pat. Nos. 4,367,924, 4,563,059, 4,813,767 and others).
In the case of manufacturing a liquid crystal panel using a ferroelectric liquid crystal, the most important subject is the development of materials for orientating the liquid crystal uniformly to obtain good electro-optic properties and good bistability. The above-mentioned U.S. patents describe methods for achieving uniform orientation by applying a strong electric field or a stress. However, since the space between the two substrates filled with liquid crystal and sealed is as narrow as 2 .mu.m or less, these methods are very difficult to practice and are thus somewhat impractical.
Meanwhile, various research efforts on separate orientation layers of liquid crystals for uniform orientation have been carried out. Various orientation layers for use in LCD devices are known. Namely, inorganic orientation layers manufactured by vacuum deposition of inorganic compounds such as SiO.sub.x (where x is an arbitrary value between 1 and 2) and organic orientation layers manufactured by forming a film of organic polymer such as polyimide and rubbing the film with a cloth can be illustrated. Recently, the Langmuir-Blodgett (LB) method was suggested. The LB methods employ a uni-molecular uni-layer or uni-molecular multi-layer of a polyimide.
However, each of the above-mentioned orientation layers have many defects. In the case of an inorganic orientation layer, productivity is lowered for mass production, since vacuum chamber equipment for orientation layer formation is needed. On the other hand, mass production is advantageously achieved in the case of an organic orientation layer, but formation of a polymer coating layer having uniform thickness is difficult and the surface of the orientation layer is contaminated owing to the electrostatic charge generated during the rubbing treatment.
In the case of using a method employing a uni-molecular LB layer (see Japanese Patent Laid-open Publication sho 62-209415, 62-211617 and others), no problem is induced by the electrostatic charge, but the mass production capability thereof is poor. That is, the polymer layer manufactured by the LB method has a uni-molecular layer which is too thin (about 4 .ANG.) and so the ITO electrodes cannot be shielded. This is undesirable for displays. Moreover, since, considering such display characteristics, the thickness of the organic orientation layer should be 500 .ANG. or more, in order to obtain a 500 .ANG.-thick layer by the LB method, about 125 layers of the 4 .ANG.-thick LB layer should be stacked. This lowers work processability remarkably, thus the LB method is not applicable in practical production.
To improve the above-mentioned defects, U.S. Pat. No. 5,067,797 discloses a method for manufacturing a uni-molecular or multi-molecular layer by a water spreading method. In this method, a polymer material is dissolved in a solvent to be spread on the surface of a body of water and is then coated on the substrate to form a thin film. The coated polymer film (polyimide (PI) or liquid crystalline polymer (LCP)) is rubbed or pressed in one direction using a roller, to form an orientation layer in which molecules align anisotropically. By this method, since the thickness of the uni-molecular layer can be somewhat controlled, the number of layer coating operations can be reduced. However, the coating efficiency is lowered and the production process for forming the orientation layer on the panel is complicated when compared with the conventional coating method. That is, this method still includes weak points in terms of processability and mass production.
Another trial for employing a liquid crystal polymer as the orientation layer is disclosed in Japanese Patent Laid-open Publication No. sho 57-40228. In this patent, contrast is improved by the elimination of disclination (discontinuous orientation) at the interface of the liquid crystal and the orientation layer by combining a side-chain mesogen with the main chain and pre-tilting the mesogen group in the orientation layer along with the liquid crystal molecules under an electric field, to thereby minimize light loss at the interface of the pre-tilted liquid crystal and the orientation layer. However, if the mesogen in the orientation layer actually moves with the liquid crystal layer according to the applied voltage, a very slow response results and high driving voltage is needed. Moreover, since the mesogen used is an alkyl mesogen, the pre-tilting becomes more difficult. Even though the liquid crystal is oriented, the side chain mesogen is liable to induce a relaxation phenomenon by a molecular fluctuation so that bistability is very difficult to maintain.
Japanese Patent Laid-open Publication sho 62-227122 discloses a method of coating a side-chain polymer used as an orientation layer and rubbing the coating in an isotropic or liquid crystal phase to achieve orientation of the layer. This method seems desirable in using the inherent properties of a liquid crystal, that is, the tendency of being changed between the liquid crystal phase and isotropic phase by external stimulation. However, actually, the orientation between the liquid crystal phase and the isotropic phase is not easy and even though orientation is obtained, orientation disruption due to the continuous stress relaxation of the side-chain polymer easily occurs.